Linear AC Motors

The first linear motor was conceived by Wheatstone more than 100 years ago. But large air gaps and low efficiencies prevented linear motors from being widely used. Though they still have relatively large air gaps, linear induction motors are increasingly chosen for material-handling applications because they are quieter, more reliable, and less expensive than rotary motors. And because linear motors do not drive gearboxes or rotary-to-linear conversion devices, they can be more efficient.

A linear motor is conceptually a rotary motor whose stator core has been cut and unrolled. The circular stator becomes a linear stator, defining a single-sided linear induction motor (SLIM). Likewise, if the circular stator is cut into two sections and flattened, the motor becomes a double-sided linear induction motor (DLIM). The DLIM and SLIM both require a two or three-phase stator (primary) winding and a flat metallic or conductive plate-type armature (secondary).

Cutting and unrolling the stator leads to many other possible linear motor configurations. For example, a tubular motor can be conceptually made from the SLIM by rerolling it in the direction of motion. The pole pattern is produced by three-phase windings in alternate clockwise and counterclockwise directions around the tube. Other designs are also possible, but few of them are used.

There are several important differences between linear and rotary induction motors that bear on selection. Unlike rotary motors, the linear motor has a beginning and an end to its travel.

First, the moving secondary material enters the primary at one end of the motor and exits at the opposite end. Induced currents in the secondary material at the entry edge resist air-gap flux buildup. And at the exit edge, the material retards the air-gap flux decay. This results in an uneven air-gap flux distribution. Such flux distribution causes little or no thrust under the first few poles at entry and a braking thrust at exit.

At stall and low speeds, the flux distribution is not seriously distorted and is usually ignored. But dynamic compensation is required to minimize thrust roll-off at high speeds.

Second, the large air gap which is endemic to linear motors effectively limits linear force. Fortunately, new pole piece designs offset the adverse air-gap effect.

The moving member in a linear motor is typically a solid conducting plate or sheet. It does not contain coils or windings. However, a linear motor can be constructed so the primary moves and the secondary remains stationary.

Linear motors also need the secondary to be wider than the primary. The secondary should be wide enough to handle induced current with little resistive losses along the transverse edge. Such losses are known as transverse edge-effects which can reduce useful thrust or force.

A normal force between stator and armature in the SLIM is perpendicular to the direction of travel. The stator and armature are either attracted or repelled by this force. Factors that determine the force direction include armature material composition and thickness, stator frequency, air gap, and pole pitch. SLIMs are constructed to minimize the normal force. For DLIMs, and rotary and tubular motors, these forces cancel.

Linear induction motors produce up to several thousand pounds of thrust. Positional accuracies of 1 ∝in. and velocities of 100 in./sec are possible. Feedback is usually from a linear encoder able to provide high resolution and accuracy. Among the encoder technologies used are optical, magnetostrictive, magnetic, and inductive.

For applications involving high accelerations, the secondary normally moves over a long stationary primary. High force and short stroke applications with low repetition rates call for a moving primary and a long stationary secondary. Conversely, for long strokes and high repetition rates, a moving secondary and long stationary primary are required.

In certain material-handling and coil-processing applications using linear-induction motors, the material itself is the secondary. For example, to handle sheet steel, overhead SLIM primaries induce currents in the steel and attract it to the primary. A balanced force is provided by gravity and an air bearing to levitate the sheets in air. The sheets are then propelled into piling zones without touching any surface or other object. Recent advances in power electronics, microprocessors, and electromagnetic analysis software are responsible for many new linear-motor designs. Power electronics provide inexpensive pulse-width modulators (PWMs), vector controllers, and variable-frequency drives. Microprocessors used for control include 32-bit processors, coprocessors, and digital signal processors (DSPs). Precision control is needed to compensate for magnetic saturation, thrust roll-off, and transverse-edge effects.